Goal: Induction of mammary tumors with biological (hormones, retroviral shuttle vectors, and mouse mammary tumor virus, MMTV) and chemical carcinogens in various feral strains of Mus musculus and other species of Mus. Breeding of transgenic strains of Mus musculus containing certain activated protooncogenes. Maintenance of preneoplastic mammary hyperplastic outgrowth lines by transplantation in syngeneic mice. Approach: Maintain a closed pedigreed colony of 1,000 feral and inbred mice. The colony is composed of approximately 700 mice that are held long term (2 years) for tumor development and 300 mice as a breeding nucleus. The breeding nucleus is composed of three pedigreed outbred colonies of feral mice having unique characteristics that are pertinent to the study of mouse mammary tumorigenesis. They are: CZECHII V-mice (Mus musculus musculus), CZECHII V+, MS (M. spretus) mice. Five transgenic mouse lines containing the MMTVLTR-Int3, WAP-Int3, MMTVLTR-Wnt-1, MMTVLTR-TGFb, and MT-TGFa transgenes are also being maintained. The TGFa "knockout" mouse strain has also recently been introduced into the colony. In addition, a limited breeding nucleus of the high-incidence C3H/OuJ, GR/imr, BALB/cfCZECHII, and BALB/cfMS inbred mouse strains and the low-incidence BALB/cp and FVB inbred mouse strains are maintained.